WO1992021948A1

WO1992021948A1 - Echelle polychromator

Info

Publication number: WO1992021948A1
Application number: PCT/EP1992/001270
Authority: WO
Inventors: Stefan Florek; Helmut Becker-Ross
Original assignee: Bodenseewerk Perkin-Elmer Gmbh
Priority date: 1991-06-06
Filing date: 1992-06-05
Publication date: 1992-12-10
Also published as: US5448351A; EP0587683A1; AU657968B2; AU1896592A; DE59201468D1; DE4118760A1; EP0587683B1

Abstract

A monochromator (14) with a prism (20) is disposed so that light passes through it before passing on to an echelle polychromator (50). The linear dispersion of the monochromator (14) can be changed by changing the angular dispersion of the prism (20). This makes it possible to investigate, at high resolution, using an echelle grating (54), a particular spectral position and the region close to it. Care is taken that, depending on the mean wavelength being observed, not only is the detector array (66) of the polychromator (50) completely used, but also interfering orders are kept away from the polychromator (50). The linear dispersion of the monchromator can be changed for this purpose.

Description

Echelle polychromator

Technical field

»The invention relates to an Echelle polychromator.

A polychromator is a spectral apparatus in which a spectrum is generated on a detector array by a dispersing element, that is to say a series of detector elements arranged close to one another. In this way, the spectral intensities at the different wavelengths of a wavelength range detected by the detector array are measured simultaneously. In contrast, a monochromator generates a spectrum in the plane of an exit slit. The light then emerges from a certain, relatively narrow wavelength range through this exit slit. The light emerging through the exit slit can then be applied to a single detector, e.g. a photomultiplier. The light falling through the exit slit can also be directed to another monochromator or polychromator. In the latter case, one speaks of a pre-monochromator. Pre-monochromators are used to reduce stray light. In the case of grating monochromators, pre-monochromators have the task of eliminating light from disturbing orders.

An Echelle grating is a grating with a triangular groove profile, the groove spacing of which is large compared to the wavelength. An Echelle grating uses high interference orders. A polychromator that works with an Echelle grating is an Echelle polychromator.

The underlying technology was

Various types of monochromators and polychromators are described in a book by J. Sternberg "The Design of Optical Spectrometers", Chapman and Hall, 1969, and in one

REPLACEMENT LEAF Y. Talmi's book "Multichannel Image Detectors", American Chemical Society, 1979.

US-A-4 820 048 describes an Echelle polychromator in combination with a new solid state detector array. There, the orders within the Echelle polychromator are separated for simultaneous detection of the entire wavelength range. This results in a limitation of the height of the entry gap. In addition, aberrations occur in the edge areas of the spectrum. The level of stray light is relatively high.

GB-A-2 204 964 describes an Echelle polychromator for multi-element analysis. A light source generates a light beam that is delimited by two crossed entry slits. The light beam falls on a collimator mirror. The collimator mirror directs the parallel light beam through a dispersion prism to an Echelle grating. The light beams diffracted by the Echelle grating pass again through the dispersion prism and are collected by a camera mirror on a two-dimensional detector array. The dispersion prism causes a dispersion in a direction perpendicular to the direction of dispersion of the Echelle grating and thus a separation of the different orders of the Echelle grating. The detector array contains detector elements at the location of characteristic spectral lines of the various chemical elements to be determined.

An Echelle monochromator is known in which the orders are separated by a pre-monochromator. The pre-monochromator generates a spectrum in the plane of an entrance slit of the high-resolution Echelle monochromator. Only a limited wavelength range from this spectrum reaches the Echelle monochromator. The Echelle monochromator then detects an even narrower wavelength range from this wavelength range with an exit slit and a detector arranged behind it. Such echelle Double monochromators achieve a high spectral resolution with extremely low stray light. The larger height of the column results in a high light guide value. However, only one spectral position can be examined within the entire spectrum (PWJM Boumans and JJAM Vrakking, "Spectrichimica Acta" Vol. 39B No. 9-11, 1239).

DE-C-3 634 485 describes a liquid prism with a variable prism angle for producing spectrally distorted photographs and projections.

Disclosure of the invention

The invention has for its object to provide a spectral apparatus which allows not only a specific spectral position but also its surroundings to be examined at high resolution at the same time.

According to the invention, this object is achieved by an Echelle polychromator, which is preceded by a pre-monochromator with a prism, the linear dispersion of the pre-monochromator being changeable.

The device according to the invention uses an Echelle grating in connection with a linear detector array, through which a multiplicity of wavelengths are simultaneously detected separately. In this respect, it is a polychromator. However, the device is not designed to detect a large wavelength range, such as the Echelle polychromator according to the aforementioned GB-A-2 204 964. Rather, the Echelle grating is used to examine a specific spectral position and its immediate surroundings with high resolution . For this purpose, the Echelle polychromator is preceded by a simple prism pre-monochromator. The pre-monochromator allows only a limited spectral range to pass through to the Echelle polychromator. But care must be taken

REPLACEMENT become that - depending on the observed mean wavelength - on the one hand the detector array is fully utilized and on the other hand disturbing orders, that is light, which would produce overlapping spectra of different order on the Echelle grating, are kept away from the Echelle polychromator. For this reason it is provided that the linear dispersion of the pre-monochromator is variable.

The invention creates a novel device that is actually an intermediate thing between a monochromator and a polychromator: the device contains a monochromator, namely the pre-monochromator, which guides a narrow spectral range through an entry slit of the Echelle polychromator. The device also contains a polychromator with a detector array. However, the detector array observes the surroundings of a set medium wavelength with high resolution, not an extended, full spectrum.

Embodiments of the invention are the subject of the dependent claims.

Embodiments of the invention are explained below with reference to the accompanying drawings.

Brief description of the drawings

Fig.l is a top view of an Echelle polychromator with prism pre-monochromator.

2 shows another embodiment of a prism with a variable angular dispersion.

Preferred embodiments of the invention

In Fig.l, a light beam 10 passes from a (not shown) light source through an entrance slit 12 Vormonochromators 14. The Vormonochromator 14 is a prism monochromator in a Littrow arrangement. The light bundle 12 falls on a parabolic collimator mirror 16. The collimator mirror 16 generates a parallel light bundle 18 from the divergent light bundle 12. The parallel light bundle 18 falls on a prism 20.

The prism 20 is a liquid mirror prism with a variable prism angle. The prism 20 contains a plane-parallel front plate 22 that is transparent to the examined light and a rear plate 24. The rear plate 24 is mirrored on its front side 26 facing the collimator mirror 16. A mirror holder 28 is attached to the rear plate 24. The mirror holder 28 has a cylindrical basic shape. The mirror holder 28 is chamfered on the plate-side end face 30. On the opposite

At the end, the mirror holder 28 forms a flange 32. A bellows 34 extends between the flange 32 and the edge of the front plate 22. Between the plate 22, the bellows 34 and the flange 32 and around the mirror holder 28

Cavity 36 formed. This cavity 36 is filled with a refractive liquid. This liquid also fills the triangular space between the two

Plates 22 and 24.

The plates 22 and 24 are pivotally connected to one another along an edge 38. In addition, the entire prism 20 with the mirror holder 28 and the bellows 34 is pivotally mounted about an axis '40. The axis 40 runs parallel to the edge 38 and to the direction of the entry gap 12. Die

Axis 40 lies in the plane of the mirrored front surface 26 of the plate 24

The refractive liquid is distilled water. Distilled water has in the entire wavelength range of

190 nm to 850 nm high transmission. The refractive index of distilled water shows only a slight temperature dependence.

REPLACEMENT LEAF The plate 22 is made of molten quartz. The bellows is made of PTFE.

The parallel light beam 18 is refracted and spectrally broken down by the prism formed by the plate 22 and the liquid located between the plates 22 and 24. The light beams thus obtained are reflected by the mirrored front surface 26 of the plate 24. The reflected light bundles pass through the prism 20 again and experience a new dispersion. The spectrally split light bundles then run back to the collimator mirror 16, as indicated by arrow 42, and are collected by the collimator mirror 16 via a deflecting mirror 44 in the plane of a gap 46. A spectrum is generated in the plane of the gap 46 as an image of the entrance gap 12. In Fig.l only the central beam of the light beam 48 is shown, which is focused on the center of the gap 46.

The beam 10 passes through the entrance slit 12 of the pre-monochromator 14 with an aperture ratio of F / 8. The beam is reflected on the parabolic collimator mirror 16 at an off-axis angle of approximately 8 °. The collimator mirror 16 has a focal length of 400 mm.

The gap 46 allows a narrow spectral range to pass through from the spectrum generated in the plane of the gap 46. The slit 46 forms the entry slit of an Echelle polychromator 50. The Echelle polychromator 50 contains a collimator mirror 52 and an Echelle grating 54. A divergent light bundle 56 with the narrow spectral range extends from the slit 46. The light bundle 56 is deflected by a deflecting mirror 58. and falls on the collimator mirror 52. The collimator mirror 52 generates a parallel light bundle 60 from the divergent light bundle 56. The parallel light bundle 60 falls on the Echelle grating 54. The Echelle grating is pivotable about an axis 62. The axis 62 is parallel to the grid furrows. The grid furrows run perpendicular to the paper plane in Fig.l.

The Echelle grating 54 breaks down the light that has entered through the entrance slit 46. The Echelle grating emits parallel light beams in different directions with the different wavelengths of the spectral range masked out by the slit. The diffracted light beams are essentially reflected back onto the collimator mirror 52 in the direction of incidence. This is indicated by an arrow 64 in Fig.l. The light beams with the different wavelengths are focused by the collimator mirror 52 in the plane of a detector array 66. A spectrum is generated on the detector array 66. The spectrum extends across the detector array 66 from left to right in Fig.l. The light bundle corresponding to the center wavelength is designated 68 in FIG. An aperture 70 shields the detector array against interference radiation. A partition 72 is provided between the pre-monochromator 14 and the Echelle polychromator 50. The entry gap 46 sits in this partition 72.

The Echelle grating 54 has a line count of 75 furrows per millimeter and a blaze angle of approximately 76 °. The Echelle grating 54 is operated close to the autocollimation in the interest of a high diffraction efficiency. The collimator mirror 52 is a parabolic mirror with a focal length of 400 mm. The detector array is a CCD line with 512 detector elements, each 0.023 mm wide and 0.480 ^' mm high. The slit width of the entry slit 12 of the pre-monochromator 14 is twice as large as the slit width of the entry slit 46 of the Echelle polychromator 50. This completely illuminates the entry slit of the Echelle polychromator 56 for all wavelengths that are to be measured with the detector array.

By adjusting the plate 22 about the axis 38 and the mirrored plate 24 (together with the entire prism 20) a narrow wavelength range is defined around the axis 40, which passes through the slit 46. The center wavelength of the region is determined by the light beam 48 which, after refraction by the prism 20, is perpendicular to the

_ mirrored surface 26 of the plate 24 is reflected. The spectral width of the wavelength range that is transmitted through the slit is smaller, the greater the linear dispersion of the pre-monochromator, ie the greater the angular dispersion of the prism 20 here. This angular dispersion can be increased by pivoting the plate 22 relative to the plate 24. The volume of liquid displaced between the plates 22 and 24 or sucked in between the plates 22 and 24 is compensated for by the elastic bellows 34.

The selected spectral range at the input slit 46 of the Echelle polychromator 50 with the selected center wavelength and the selected spectral width is now split by the Echelle polychromator with high dispersion, and a corresponding spectrum is generated on the detector array 66. For this purpose, the Echelle grating 54 is pivoted in such a way that the center wavelength lies approximately in the middle of the detector array 66. The angular dispersion of the prism 20 and thus the spectral width of the spectral range is chosen so that the length of the detector array 66 is optimally used. For this purpose, the angular dispersion of the prism 20 is chosen so that the following condition is met:

2sg (sino. + Sinß) / (f ₁ λ ² ) <d <T / dλ <sf ₂ (sin «<+ sinß) / (l flλcosß).

where d <f / dλ is the angular dispersion of the prism, λ is the wavelength, s is the width of the entrance slit of the Echelle polychromator, g is the lattice constant of the Echelle grating of the Echelle polychromator, «, ß angle of incidence or diffraction at the Echelle grating, f ^ is the camera focal length of the pre-monochromator, f _{2 is} the camera focal length of the Echelle polychromator and 1 is the length of the detector array of the Echelle polychromator. In this way it is ensured that the length of the detector array 66 is fully utilized, that is to say the wavelength range required for complete illumination of the detector array reaches the Echelle polychromator through the entrance slit '46, and on the other hand this spectral range is smaller than the free wavelength range of the order used of the Echelle grating 54, so that there is no overlap of orders.

The Echelle polychromator with pre-monochromator can be modified in various ways.

Fig. 2 shows schematically a modified form of the prism with variable angular dispersion for the pre-monochromator.

The prism 68 consists of two solid bodies 70 and 72 made of a transparent, dispersing material in the wavelength range used, e.g. Quartz. The body 70 has a flat front surface 74. On the back, the body 70 has a concave-cylindrical surface 76. The body 72 has a convex-cylindrical front surface 78 which is complementary to the surface 76. The surfaces 76 and 78 abut one another in such a way that rays pass through the separating surface essentially smoothly. If necessary, a liquid can be introduced between the surfaces 76 and 78, the refractive index and dispersion of which have essentially the same values as the material of the bodies 70 and 72. The body 72 has a flat rear surface 80. The surface 80 is provided with a mirror 82 Mistake. By pivoting the two bodies 70 and 72 against each other, the prism angle between the surfaces 74 and 80 can be changed continuously.

Instead of cylindrical surfaces e.g. spherical surfaces can also be provided.

REPLACEMENT BL The change in the linear dispersion of the pre-monochromator could also be achieved by changing the focal length of the imaging optics. A zoom lens would then have to be provided.

Claims

1. Echelle polychromator (50), which is preceded by a pre-monochromator (14) with a prism (20), the linear dispersion of the pre-monochromator (14) being changeable.

2. Echelle polychromator according to claim 1, characterized in that the angular dispersion of the prism (20) can be changed to change the linear dispersion of the pre-monochromator (14).

3. Echelle polychromator according to claim 2, characterized in that the prism angle of the prism (20) is variable.

4. Echelle polychromator according to claim 3, characterized in that the prism (20) has a front plate (22) and a rear plate (24) which form a variable angle and enclose a cavity filled with liquid between them.

5. Echelle polychromator according to one of claims 1 to 4, characterized in that the pre-monochromator (14) is a prism monochromator in a Littrow arrangement with a Littrow mirror (24, 26).

6. Echelle polychromator according to claims 4 and 5, characterized in that the rear plate (24) of the prism (20) is formed reflective to form the Littrow mirror.

7. Echelle polychromator according to claim 5 or 6, characterized in that the Littrow mirror (24,26) and the front plate (22) are adjustable together so that

REPLACEMENT LEAF an entrance slit (46) of the Echelle polychromator (50) is passed through a spectral range with a selectable center wavelength, the angular dispersion of the prism (20) depending on the selected wavelength being chosen such that on the one hand a detector array (66) of the Echelle polychromator (50) optimally illuminated and on the other hand an overlap of orders of the spectrum generated by the Echelle polychromator (50) is excluded.

8. Echelle polychromator according to claim 7, characterized in that the angular dip version of the prism (20) for a predetermined wavelength of the condition

2sg (sinc <+ sinß) / (f ₁ -_.-) <d5 ^" / d \ <sf ₂ (siι_oC + sinß) / (l flλcosß)

are sufficient, where a / d is the angular dispersion of the prism, λ. the wavelength, s the width of the entrance slit of the Echelle polychromator, g the grating constant of the Echelle grating of the Echelle polychromator, o, ß angle of incidence or diffraction at the Echelle grating, f ^ the focal length of the camera of the pre-monochromator, f ₂ the focal length of the camera Echelle polychromator and 1 is the length of the detector array of the Echelle polychromator.

9. Echelle polychromator according to claim 2, characterized in that the prism (68) consists of two solid parts

(70, 72), one of which forms the flat front surface (74) and the other the flat rear surface (80) of the prism (68) and the concave-cylindrical or complementary convex-cylindrical, facing each other have adjacent surfaces (76, 78) permitting a relative rotation of the two parts (70, 72).

10. Echelle polychromator according to claim 2, characterized in that the prism consists of two solid parts, one of which forms the flat front surface and the other the flat rear surface of the prism and the facing concave-spherical or complementary 15

to this end have convex-spherical, abutting surfaces which permit relative rotation of the two parts.

11. Echelle polychromator according to claim 1, characterized in that for changing the linear dispersion of the pre-monochromator, the focal length of an imaging optics of the pre-monochromator can be changed.

12. Echelle polychromator according to claim 8, characterized in that an entry gap (12) of the pre-monochromator (14) has twice the width as the entry gap (46) of the Echelle polychromator (50).

REPLACEMENT LEAF